There are several types of crucibles that can be used for heating up materials. Possible types of crucibles are alumina, ceramic and porcelain. Most labs require the use of porcelain crucibles. Porcelain crucibles are glazed on the outside and inside, except there is usually no glaze on the bottom.
Porcelain Crucibles are used with Muffle Furnaces for loss-on-ignition (LOI) testing of cement, fly ash, and soil materials.
They are temperature-rated to 1,832°F (1,000°C). They have a high-form
design and are glazed inside and out, except for the outside bottom and
rim.
http://www.cscrucible.com/csceramic/product/view_1382.html
https://www.ceramicmanufacturing.net/ceramic-crucibles/
https://www.ceramicmanufacturing.net/alumina-crucibles/